The present invention relates generally to signal generating circuits in colour television systems and more particularly to a signal generating circuit which is used in colour television cameras and the like and, with a simple circuit organization, is capable of generating chrominance sub-carrier wave signals, horizontal scanning frequency signals, vertical scanning frequency signals, and the like of PAL systems and SECAM systems.
In general, in a colour television camera, a colour television game device, or the like, there is provided a circuit for generating signals such as a chrominance sub-carrier wave signal, a horizontal synchronizing signal, and a vertical synchronizing signal.
In general, the frequency fsc of the chrominance sub-carrier wave of a colour television video signal of a PAL system is selected at a value expressed by the following equation in terms of the horizontal scanning frequency fh (=15,625 Hz) and the vertical scanning frequency fv (=50 Hz). EQU fsc=(284-1/4)fh+fv/2=4,433,618.75 Hz (1)
The chrominance sub-carrier wave frequency fsc and the horizontal scanning frequency fh have the following mutual relation. EQU 709,379 fh=2,500 fsc (2)
That is, in a PAL system, as is indicated in Eq. (1), a frequency offset of fh/4 is provided in order to cause the chrominance sub-carrier wave to undergo interleaving with the horizontal scanning frequency, and a frequency offset of fv/2 is set in order to reduce cross colour to an inconspicuous degree. For this reason, even when the chrominance sub-carrier wave frequency fsc or a frequency value which is an integral multiple thereof is frequency-divided in a simple manner, the horizontal scanning frequency fh and the vertical scanning frequency fv cannot be obtained.
Accordingly, a signal generating circuit known heretofore has had an organization comprising a reference signal generator for generating a signal of a frequency of 284 fh, a pulse generator for frequency-dividing and wave-shaping the signal from the reference signal generator and generating signals of the horizontal scanning frequency fh and the vertical scanning frequency fv, a 1/4 frequency divider for 1/4 frequency-dividing the signal of the horizontal scanning frequency fh thus generated, a 1/2 frequency divider for 1/2 frequency-dividing the signal of the vertical scanning frequency fv thus generated, a first frequency subtraction circuit for carrying out subtraction of the signal of frequency fh/4 from the 1/4 frequency divider and the signal of frequency fv/2 from the 1/2 frequency divider, and a second frequency subtraction circuit for carrying out subtraction of the signal of frequency 284 fh from the reference signal generator and the resulting signal of frequency (fh/4-fv/2) from the first frequency subtraction circuit. From the second frequency subtraction circuit, a chrominance sub-carrier wave signal of the PAL system of a frequency of 284 fh-( fh/4-fv/2)=(284-1/4)fh+fv/2 is obtained.
For each of the frequency subtraction circuits mentioned above, a double balanced modulator or a circuit comprising a combination of a balanced modulator and a single-sideband filter is generally used. A frequency subtraction circuit of such an organization, however, has been accompanied by the problem of complicated circuit organization. Furthermore, since the frequency of a leakage carrier (upper sideband, etc.) included in the output of a balanced modulator is close to the required output frequency, it is necessary to connect a band-pass filter having a steep cutoff characteristic to the output side of the balanced modulator. Then, since the frequency value represented by (fh/4-fv/2) is low, the cutoff frequency of this band-pass filter must be low. It has been difficult to produce at low cost a band-pass filter having such a low value of cutoff frequency and, moreover, a steep cutoff characteristic.
Thus, the above described known signal generator employs two frequency subtraction circuits of complicated circuitry and high cost, and for this reason, the entire circuit has been of complicated organization and high price.
One the other hand, in order to obtain a signal generating circuit for a
system of inexpensive organization, omission of the above described first frequency subtraction circuit and omission of the offsetting of the frequency of fv/2 has been tried in the prior art. However, in a signal generating circuit of such an organization, the frequency relationship departs greatly from that of Eq. (1), whereby the performance deteriorates markedly, and the circuit cannot be used in actual practice.